The present invention relates to a device for optically measuring temperature employing the phenomenon of variation of the photoabsorption of a semiconductor optical crystal with temperature. Such a device for optically measuring temperature is generally required to be capable of measuring temperature over a wide range and to be reliable within the range of temperature to be measured over long periods.
Devices for optically measuring temperature heretofore proposed may be broadly divided into two classes, a first wherein an optical fiber is connected between the object of which the temperature is to be measured and an infrared sensor, and a second, termed a contact type, which includes a detection element, for which an optical transmission property varies with temperature, disposed in contact with the object and and arranged to modulate a signal light beam carried by an optical fiber.
The temperature measuring device of the first type is suitable for measurement of high temperatures since it generally is usable in a temperature range of more than about 500.degree. C. However, such a device is not usable below about 500.degree. C., and hence cannot be applied to most industrial temperature measuring applications.
The temperature measuring device of the second type has been known to include four different types of detecting elements. These include (a) a bimetallic or thermocouple element, (b) a double-refractive crystal, (c) a liquid crystal having a temperature-dependent refractive index, and (d) a phosphorescent material having a white absorption characteristic which varies with temperature. However, temperature measuring devices which employ such materials suffer from drawbacks in thermal stability and mechanical stability. Also, such devices are generally limited to use at a maximum temperature of about 350.degree. C.
In a further attempt to provide a temperature measuring device which performs satisfactorily at moderate and low temperatures, it has been proposed to employ a semiconductor or compound semiconductor material as a contact-type detector. In such an approach, a light beam from a semiconductor laser passes through the temperature detecting element in an amount determined by the temperature of the detecting element. Optical absorption and wavelength characteristics of the temperature detecting element are selected to correspond to those of the semiconductor material of the light source. More specifically, for an AlGaAs semiconductor laser emitting light at about 0.8 microns, temperature detecting elements fabricated with combinations of GaAs or CdTe have been used. However, due to limitations imposed by the spectral width of the light source, the maximum temperature which can be measured using such a device is about 200.degree. to 300.degree. C. Moreover, such compound semiconductor materials have a solid solution phase which is present at temperatures of above about 300.degree. C. which means that such materials do not have the desired thermal stability.
Accordingly, it is a primary object of the present invention to provide a temperature measuring device which is free from the drawbacks of prior art devices.
More particularly, it is an object of the present invention to provide a temperature measuring device which is capable of measuring temperatures over a wide range, yet which is reliable and stable over a wide range of temperatures and over long periods of time.